I think you need to do that via the python layer's transaction parameters.

something like: (from the docs)
customTPB = TPB()
customTPB.lock_timeout = 5

# Explicitly start a transaction with the custom TPB:
con.begin(tpb=customTPB.render())

see:
http://www.firebirdsql.org/devel/python/docs/3.3.0/beyond-python-db-api.html#advanced-transaction-control